1. Field of the Invention
The present invention relates to an optical scanning apparatus used for an image forming apparatus such as an electrophotographic copying machine or a printer and an image forming apparatus including the same.
2. Description of the Related Art
An optical scanning apparatus included in an electrophotographic image forming apparatus includes a light source for exposing a photosensitive drum and a rotational polygonal mirror (hereinafter referred to as a polygonal mirror) for deflecting and scanning a laser beam emitted from the light source. The laser beam emitted from the light source is deflected to scan by the rotating polygonal mirror. The laser beam that has been deflected to scan by the polygonal mirror is used for scanning the photosensitive drum in a direction parallel to a rotational axis of the photosensitive drum. An electrostatic latent image is formed on the photosensitive drum charged by a charging unit with the scanning laser beam. The electrostatic latent image is developed by a toner, and a toner image on the photosensitive drum is transferred on a recording medium so that an image is formed on the recording medium.
When the polygonal mirror rotates, a steady air current is generated. There are broadly two kinds of air current. One is an air current departing from the polygonal mirror and another is an air current directed toward the polygonal mirror. Examples of the air current departing from the polygonal mirror include an air current generated by pressing air against a reflection surface of the polygonal mirror, and an air current entrained and generated around the polygonal mirror toward the polygonal mirror due to the viscosity of the air. The air currents are respectively directed in a radial direction and a tangential direction of a circumscribed circle of the polygonal mirror.
On the other hand, the air current toward the polygonal mirror flows into a space in which the air current departing from the polygonal mirror causes the air to flow out and the atmospheric pressure decreases. As described above, air in the vicinity of the polygonal mirror flows out as the air current departs from the polygonal mirror when the polygonal mirror rotates. The air current directed toward the polygonal mirror from above the polygonal mirror is generated to replenish the air that has flowed out.
When the polygonal mirror is driven to rotate, heat is generated in a motor or an integrated circuit (IC) of a driving device for rotating the polygonal mirror. The heat causes the air current departing from the polygonal mirror to heat up. The air current that has heated up applies heat to an optical member (hereinafter referred to as an optical lens) arranged in the vicinity of the polygonal mirror. Therefore, the optical lens is thermally deformed, and optical performance thereof is deteriorated.
On the other hand, Japanese Patent Application Laid-Open No. 2007-79515 discusses an optical scanning apparatus for reducing thermal expansion of an optical lens by preventing an air current departing from a polygonal mirror from striking the optical lens, as much as possible. In Japanese Patent Application Laid-Open No. 2007-79515, an air shield member which has an opening for passing a laser beam is provided between the polygonal mirror and the optical lens. This prevents an air current generated by the rotation of the polygonal mirror from striking the optical lens except in the opening, thus deformation of the lens can be reduced.
However, the optical scanning apparatus discussed in Japanese Patent Application Laid-Open No. 2007-79515 presents the following issue when miniaturized. In order to miniaturize the apparatus, distances among members installed in an optical box must be reduced. As an example, a distance between a polygonal mirror and an optical lens may be reduced. In this case, the optical lens is arranged in the vicinity of an opening for passing a laser beam. In such an apparatus, the air current can be cut off by providing a wall (an air shield member) between the polygonal mirror and the optical lens, as in Japanese Patent Application Laid-Open NO. 2007-79515. However, hot air strikes the optical lens before the temperature of the air current that has passed through the opening decreases. Therefore, the hot air that has passed through the opening deforms the optical lens.
If the distance between the polygonal mirror and the optical lens is great, the air current that has passed through the opening is cooled to some extent before striking the optical lens. Therefore, an amount of deformation of the optical lens due to the effect of the hot air is small. When the distance between the polygonal mirror and the optical lens is short, as in the above-described apparatus, however, the air current strikes the optical lens before being cooled, and the air current flowing out of the opening has difficulty passing in the vicinity of the optical lens. Therefore, the amount of thermal deformation of the optical lens increases and its effect on image quality cannot be ignored.
When the optical lens is thermally deformed, the spot diameter of a laser beam is disturbed, and an image forming position on the photosensitive drum of the laser beam is shifted, so that the image quality is deteriorated.